The use of chemical reactions, especially involving a heterogeneous catalyst, in fixed bed reactors is known. In processes of this type, control of the heat absorbed or emitted represents a critical aspect.
For example, in exothermic reactions such as combustion reactions, a hot front has a tendency to form in the reactor when the reaction starts, and this hot front has a tendency thereafter to move in the bed of the reactor in the flow direction, until optionally exhibiting a risk of leaving the bed.
In order to resolve this problem, reverse-flow reactors have been developed. The principle of these systems is based on a reversal of the reaction flow that makes it possible to keep the hot front in the bed of the reactor during the reaction.
Documents WO 02/051965 and U.S. Pat. No. 5,710,356 provide examples of such symmetrical systems, in which the same flow passes through the reactor alternately in one direction and in the opposite direction.
Document U.S. Pat. No. 7,763,174 describes a variant in which adsorbent beds are placed on each side of the catalyst bed.
Still with an objective of optimizing heat transfers, asymmetric reverse-flow reaction processes have also been proposed, in order to enable the thermal coupling of an exothermic reaction and of an endothermic reaction. For example, a first flow direction may be used in order to carry out the combustion of methane (injection of methane and air), whilst the reverse flow direction may be used to carry out methane reforming (injection of methane and water). The first reaction is intended to heat the catalyst bed to a sufficient level to enable the second, endothermic, methane reforming reaction. Document US 2004/0170559 provides an illustration of such an asymmetrical process.
In the reverse-flow reactor systems described above, no specific measure is provided for the regeneration of the catalyst, once the latter is deactivated.
Document DE 10239547 illustrates another example of a thermal coupling of an endothermic reaction and of an exothermic reaction, this time by combining methane reforming with catalyst regeneration. According to this process, steam methane reforming is carried out at 400° K in a reactor comprising distributors for the injection of a supplementary flow introduced by supply pipes integrated into the reactor. During the regeneration, the flow direction is reversed and the internal distributors are supplied with a fuel in order to reach a regeneration temperature of 1000° K.
The process performed in document U.S. Pat. No. 4,461,745 consists in simultaneously carrying out a reaction and a regeneration, each in one half of a reaction chamber, then in reversing these 2 phases, the stream resulting from the regeneration step always being mixed in the reaction chamber with the reaction stream in order to supply the reaction phase zone.
Within the context of catalytic reactions for fluorination or for production of acrylic compounds that are carried out in a fixed bed, the catalyst is deactivated in particular by elimination of coke thereon. The regeneration of the catalyst may be carried out by combustion of the coke, by injecting a stream rich in oxygen or in air into the reactor.
However, the proportion of coke in the reactor is not homogeneous: there is, in principle, more coke accumulated toward the inlet of the reactor than toward the outlet thereof. This poses a combustion control problem. For example, a hot front may appear, leading to a very high temperature differential, which is capable of degrading the catalyst.
There is therefore a need to achieve a better control of the regeneration phase of processes of this type, in particular by avoiding the risks of degradation of the catalyst.